in the rain shadows of mountains, such as california's sierra nevada. the rain shadow effect works as follows. warm, moist air moves east across the pacific. it hits the coast and is forced upward to get over the mountains. as it rises, it expands and cools, and its moisture turns into clouds, rain, and sometimes snow. this leaves the air dry. as this cool, dry air moves down, on the leeward side of the mountains, it is compressed and heats up again. the air, now warm as well as dry, sucks up what little moisture may be available from the land below, creating deserts as it continues its journey east of the sierra nevada. many of the world's most prominent or well-known deserts are the result of this rain shadow effect-- the mojave in the united states, the gobi desert in central asia. western south america has a similar situation. we have a chain of mountains down the west coast-- the andes-- which serve as a barrier forming this rain shadow. another factor that plays an indirect role in the formation of deserts is plate tectonics. the position of the continents in the polar regions or the eq

sculpted by repeated glaciation. yosemite valley, here in the sierra nevada mountains, would have been another nondescript river valley if glaciers hadn't carved it to its present shape. many of the world's most beautiful lakes were gouged out of hard rock by glaciers, including north america's great lakes and the famous lochs of scotland. even the great expanses of rich agricultural soils that blanket china and the soviet union, canada and the united states owe their existence to glaciers. moving glacial ice pulverizes the underlying rock into silt-sized fragments. this silt was eventually transported and concentrated by the wind into the vast fertile soils of today. early scientists didn't really appreciate the important geological role of glaciers. even geologists were convinced that glaciers had never existed outside of their present locations over the last one million years. a breakthrough came in 1836 when swiss scientist louis agassiz reported evidence that the inhabitants of medieval villages in europe had moved their towns to keep pace with advancing glaciers. further study re

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